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Avalanche Forecast

Archived

Feb 21st, 2021–Feb 22nd, 2021

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Lizard-Flathead.

A storm will impact the region Sunday night into Monday, forming new slabs, destabilizing the snowpack, and increasing the likelihood of triggering buried weak layers. An avalanche cycle may occur at all elevation bands. Travel in avalanche terrain is not recommended.

Confidence

Moderate - Uncertainty is due to the track & intensity of the incoming weather system.

Weather Forecast

SUNDAY NIGHT: Cloudy with snowfall, accumulation 10 to 20 cm with locally more snowfall possible, 50 gusting to 100 km/h southwest wind, alpine temperature -5 C, freezing level 1300 m.

MONDAY: Cloudy with snowfall, accumulation 5 to 15 cm, 40 to 80 km/h southwest wind, alpine temperature -5 C, freezing level 1600 m.

TUESDAY: Cloudy with snowfall, accumulation 5 to 10 cm, 30 to 40 km/h west wind, alpine temperature -8 C.

WEDNESDAY: Clear skies, 20 km/h southwest wind, alpine temperature -9 C.

Avalanche Summary

Many human-triggered avalanches have been triggered on the buried weak layer described in the snowpack summary. Check here for the most recent event and otherwise scroll through the loads of MINs on the map or list view. In general, the avalanches were small to large (size 1-2), around 40 to 70 cm thick, most commonly occurring at treeline elevation but also below treeline in open trees, and they have released on surprisingly low angle terrain.

Looking towards Monday, an avalanche cycle may occur if the forecasted precipitation occurs, with storm, wind, and persistent slabs at treeline and alpine elevations and loose wet avalanches below treeline. It will be a good day to avoid avalanche terrain!

Snowpack Summary

Somewhere around 20 to 50 cm of snow is forecast to accumulate by Monday afternoon. The freezing level is forecast to be up to 1600 m so rain will soak the snowpack below and storm slabs are expected to rapidly build above. The wind is forecast to be strong to extreme, meaning wind slabs will also form in exposed terrain. This is all a recipe for an avalanche cycle!

The snow and rain will load a weak layer found around 50 to 100 cm deep. The layer may consist of weak and feathery surface hoar, sugary faceted grains, or faceted grains associated with a hard melt-freeze crust. Many avalanches in the past few weeks have been triggered on these layers.

There are presently no deeper concerns.

Terrain and Travel

  • Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.
  • Travel in alpine terrain is not recommended.
  • The trees are not the safe-haven they normally are at this time. Terrain at treeline is primed for human triggered avalanches.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.